Switch machines are used to move a portion of track at a switch point in a railway system to switch a train from one track to another. It is commonplace for personnel controlling switch machines to be located hundreds or even thousands of miles away from the locations of switch points at which the switch machines are installed such that they cannot observe the operation of the switch machine with their own eyes. Such personnel must remotely control such switch machines via control signals sent to those locations, and they must rely on indicator signals sent back from sensors at those locations to tell them when a switch machine has completed a given track switching operation.
Such remote operation, therefore, makes the reliability of switch machines and the ability to be certain of the status of the tracks at switch points at any given time of great importance. Of all of the possible scenarios for a malfunction of a switch machine, the one that railroad operators most wish to avoid is a control malfunction causing the switch machine to suddenly change the position of a portion of track at a switch point just at the moment where a train is approaching the switch point such that it is too late for the train to stop before reaching the switch point with the result that the train is derailed.
As is known in the art, electromechanical switching devices (vital relays or contactors) are used for the control of switch machines. These devices are of such a design and construction to preclude malfunction and related movement of points. Erosion of contacts in a relay employed in current switch machines can occur when arcing takes place between a moving contact and a stationary contact as the moving contact moves into or out of engagement with the stationary contact. Contact erosion is the result of there being a large amount of electrical current being switched by the relay, which is the case in a switch machine since the motor required to move a portion of track between two switch positions is typically a large motor requiring a great deal of power. With current state of the art there is no mitigation of arcing and resultant contact erosion and thus the switching devices must be replaced periodically. However, the controls are consistent with prevention that could cause the points to move opposite to the intended direction. Although indicator lights at the switch point will warn the train engineer operating the train that the switch point has suddenly started moving the portion of track again, trains are typically unable to stop very quickly, and the train engineer may not be able to stop his train soon enough to avoid derailment or collision. Therefore it is imperative to maintain that security with any new control scheme.
Another issue affecting reliability of switching machines is an occasion in which the movement of a portion of track from one switch position to another cannot be completed because of either a mechanical malfunction or an obstruction preventing the portion of track from moving to the new switch position. In such situations, there is the risk of damaging the motor of the switch machine if the motor is allowed to continue struggling to move the portion of track. It is typical to employ a second relay configured to cut the power to the motor in such a circumstance. A resistor with a high temperature coefficient is coupled in parallel with the actuating coil of the second relay that causes the second relay to trip in response to the motor suddenly drawing more current for a protracted time. Such a use of a relay is effective, but adds considerably to the cost of the switching machine.